The present invention relates generally to storage batteries, and in particular, to an improvement in the manufacture of dry charge batteries.
For a variety of reasons, it is often desirable to produce batteries in a dry, fully charged state. Primarily, this is because such batteries are capable of being stored for prolonged periods of time before activation for use by the addition of battery acid. Dry charge batteries are also more easily shipped and stored since, when in their dry state, such batteries are relatively light in weight and more safely handled than are comparable batteries containing battery acid. Since dry charge batteries retain their charge in prolonged storage, this process is particularly useful in connection with batteries for which turn around time is very slow, such as industrial batteries.
As is well known in the art, the positive plates of such batteries present little difficulty in manufacture since they can be dried in the open air without harmful effect. However, the negative plates of such batteries tend to react with oxygen in the air during drying after the bulk of the water has been removed. During this stage of drying the sponge lead in the negative plates oxidizes fast enough to give off steam. This results in oxidation of the negative plates, leaving them inactive and rendering them useless.
To prevent oxidation of the negative plates during drying, it is generally necessary either to exclude oxygen from the drying atmosphere or to dry the plates in the presence of a suitable oxidation inhibitor. Both methods have been attempted in the prior art, with varying degrees of success, and at varying cost.
It has been proposed, for example, to exclude oxygen from the negative plates by drying them with steam or kerosene. Oxygen may also be excluded by drying the plates in a vacuum. This is, of course, an expensive procedure.
Another method of protecting the negative plates against oxidation during drying is to dry them in an atmosphere of carbon dioxide. In such case, the carbon dioxide reacts with the surface layer of lead to form a water insoluble (0.0011 grams/liter) layer of lead carbonate which protects the underlying lead from oxidation, but which readily decomposes in battery acid. Such protection is analogous to the well-known passivation of highly active metallic aluminum by the formation of an insoluble layer of aluminum oxide to prevent further oxidation. When negative lead plates passivated with carbon dioxide are contacted with battery acid, the lead carbonate layer is converted back to lead sulfate, with the evolution of carbon dioxide, leaving the plates in activated condition. This method of passivation is also desirable in that it does not poison the plates. However, a disadvantage is that such passivated plates are not waterproof, and therefore tend to absorb water from the atmosphere and slowly lose their electrical charge under humid storage conditions.
Chemical inhibitors have also been employed to prevent or reduce the rate of oxidation of negative plates during drying. For example, formed and washed negative plates may be soaked in a hot solution of boric acid and then dried in air. However, this method requires high concentrations of boric acid, i.e., 5 to 10%, and is only marginally effective in reducing oxidation during drying. It has been found, moreover, that boric acid treatment reduces the cranking performance of the plates on cycling. Another serious disadvantage is that boric acid does not waterproof the plates, with the result that they tend to absorb water and lose their charge, particularly when stored under humid conditions.
Another chemical inhibitor which has been employed is 1-hydroxy-2-naphthoic acid. However, again, relatively high concentrations (approximately 10% by weight) are required for use in aqueous solution. For this reason, this constituent has generally been added to the lead paste which is used in manufacturing the negative plates to enable the use of lower concentrations (approximately 1% by weight). In either case, in view of its cost, this constituent is not presently considered commerically attractive.